I keep a copy of the stamp in the compartment of my grip (that has a hinge door). I also keep a copy of all my stamps in my glove box. They are also scanned and saved in my gmail. Some ranges want to see AND copy them to have on file, others have just asked to see the stamps, and one said " the less we know the better" and told me they never want to see them (aimhi).
As far as resale.
Factory SBR guns seem to cost a lot more than their carbine version. You actually use a form 4 for this. And you DO NOT have to engrave the gun. but, you also cannot shoot is as a pistol before the stamp clears... It has to stay at the dealer the entire time. Resale for this item shouldn't be too hard, but... It's a much smaller niche within a niche.
A built SBR is on a form 1, you can build a pistol and shoot it while you wait. But , YOU MUST engrave the lower receiver. And, if you ever get tired of it you can sell it (tough because your name is engraved on it). You can put a 16 inch barrel on it and inform the ATF and then sell it to anyone freely ( but it still has your name on it). Finally, you can take your lower off and keep it. Buy a new stripped lower for 60-75 and put it on the "SBR parts" less the stock and sell it as a pistol, OR... Take the new lower, add a 16 inch barrel and sell it to anyone.
In final, SBRs aren't the type of guns you should worry about resale for.